SEOUL, South Korea — For the fourth straight weekend, masses of South Koreans were expected to descend on major avenues in downtown Seoul demanding an end to the presidency of Park Geun-hye, who prosecutors plan to question soon over an explosive political scandal.

Police expected about 50,000 protesters to turn out on Saturday near City Hall and a boulevard in front of an old palace gate, where hundreds of thousands a week before marched in what may have been the country's largest demonstration since it shook off dictatorship three decades ago. Organizers anticipated a crowd as large as half a million.

It will be the latest in a wave of demonstrations calling for the ouster of Park, who critics accuse of undermining the country's democracy by allegedly allowing a secretive confidante to manipulate power from the shadows and amass an illicit fortune.

Park's supporters were planning to hold smaller counter protests in nearby streets around Seoul Station, raising concerns about potential clashes with the anti-Park activists whose gatherings in recent weeks have been peaceful.

Prosecutors plan to formally charge Park's friend, Choi Soon-sil, by Sunday, and also are seeking to question Park in the next few days.

Choi, the daughter of a late cult leader who emerged as Park's mentor in the 1970s, has been suspected of interfering with state affairs despite having no government job, and, with the help of presidential aides, bullying companies into donating tens of millions of dollars into foundations she controlled.

Prosecutors also are seeking to indict two of Park's former aides who allegedly conspired with Choi. Other key suspects include a music video director who supposedly used her ties with Choi to win lucrative government culture projects, and a former vice sports minister suspected of providing business favors and financial support to sports organizations controlled by Choi and her niece.

Emboldened by the huge protests in recent weeks, opposition parties have been stepping up pressure to force Park to quit.

On Thursday, they used their parliamentary majority to pass a law that would allow for a special prosecutor to investigate the scandal and potentially expose the president's wrongdoings.

Opposition parties have yet to seriously push for Park's impeachment over fears of triggering a backlash from conservative voters and negatively impacting next year's presidential election.

However, there is a growing voice within the opposition that an impeachment attempt is inevitable because it's unlikely Park will resign and give up her immunity from prosecution.

Park's term lasts until Feb. 24, 2018. If she steps down before the presidential vote on Dec. 20, 2017, an election must be held within 60 days.